In a typical paper manufacturing process, a paper web is created on a paper machine and wound onto a large roll called a parent roll. The paper web is then unwound from the parent roll and converted into consumer sized products on a converting line. In paper manufacturing, as in many manufacturing processes, efficient operations that maximize operational time are desired. Defects may occur, however, in the paper web as it is being manufactured on the paper machine. These defects may be significant enough to cause the paper web to break while, for example, the web is being unwound on the converting line. A web break reduces productivity in the converting line, because an operator must stop the converting line in order to re-thread the paper web. This process may take from about five minutes to about an hour. At typical converting speeds of about two thousand feet per minute, each web break reduces the amount of paper product produced by about ten thousand feet up to about one hundred twenty thousand feet. It is, therefore, desirable to accurately identify these web defects and take action on the converting line to prevent web breaks from occurring.
The inspection of a paper web while it is being created on a paper machine is commonly performed in the art. There are also many patents, such as U.S. Pat. No. 6,452,679, directed towards web inspection. Inspection of the paper web on the paper machine is commonly used to provide real-time feedback for the papermaking process. In this way, the paper machine can be adjusted to minimize the generation of defects or to adjust other parameters of the paper web, such as basis weight.
The defect information from the web inspection may also be used to repair or to remove the portions of the paper web having the defect, before these portions result in a web break on the converting line or a failure during operation. In U.S. Pat. No. 6,934,028, a paper web is inspected, and defects are classified and located relative to periodically placed fiduciary indicators. Using these fiduciary indicators, a portion of the web having a defect may be identified and removed. Similarly, in U.S. Pat. No. 7,297,969, a paper web is inspected, periodically marked, and wound on a reel. This patent discloses a mark sequence in which the spaces between the starting points of adjacent marks are used to encode a location along the length of the web. These marks may then be used to locate defects on the paper web that were identified during inspection. The paper web is placed on a repair machine and the reel is unwound. The marks are used to stop the unwinding at a defect location so that the defect may be repaired. While not using a repair machine, U.S. Pat. No. 6,725,123 likewise discloses using marks to stop a converting line, so that a defect may be repaired. U.S. Pat. No. 8,060,234 discloses a method and an apparatus similar to that discussed in U.S. Pat. No. 6,725,123. But, instead of using marks to subsequently identify a location on a paper web on a converting line, U.S. Pat. No. 8,060,234 discloses using an optical signature for one lane of the paper web. The optical signature is the small-scale and large-scale variability inherent in a paper web.
In another method known in the art, defects are identified during web inspection and located based on their position relative to one end of the paper web. The position of the paper web may be located as a function of the diameter of a parent roll. A laser may then be used to measure the diameter of the parent roll as it is unwound, in order to locate a defect on the paper web. While the laser may be very precise, small out-of-round conditions on the parent roll may have a large impact on the position of the paper web as measured by the laser. Accordingly, this method has a large uncertainty.
In another method, a web defect is marked with a physical tag, such as a tag disclosed in U.S. Pat. No. 5,415,123. This method is heavily reliant on operator skill and expertise, because it requires the operator to observe the tag and to take action to stop the converting line in a sufficient amount of time to prevent the defect from causing a web to break.
A series of patents, for example, U.S. Pat. Nos. 7,937,233; 8,175,739; and 8,238,646, discloses a system in which a paper web is inspected for defects and periodically marked with “fiducial marks.” This system then creates a defect map where defects identified during the inspection are mapped relative to the fiducial marks. These defect maps are then used to apply locating marks at the position of the defects. Because the paper web is cut into smaller sections, a converting plan can be created to more effectively utilize the paper by cutting around the defects. Further, the defect maps may be used to sort the paper web into different grades of paper.
Each of these methods treats the defects individually and establishes other individual action points to stop and to repair or to discard a portion of the paper web. There is thus a need for improved methods and systems for defect identification, marking, and converting line control.